Plant hormones are chemicals that are present in the living body of a plant and bring about a variety of physiological phenomena. A plant hormone which is called “brassinosteroid” has a variety of physiological effects on plants, including promotion at the cellular level such as promotion of cell elongation or cell division, regulation at the organ level such as opening of cotyledons, hypocotyl elongation, differentiation of vascular bundle, epinasty of green leaves, petiole elongation, or stem elongation, imparting of environmental stress tolerance such as cold tolerance, salt tolerance, and desiccation tolerance, and promotion of plant disease resistance through plant innate immune activation. Thus, it is effective to give brassinosteroid to plants for promotion of plant growth, crop yield increase, enhancement of tolerance to environmental stress, and the like. However, although brassinosteroid is recognized to have useful effects as described above in development of breeding of crop, etc., it has not been actually used in agriculture and plant biomass production in large quantities because it is expensive and problematic in terms of penetration into plants, migration, stability, etc.
In addition, although the most part of research on biosynthesis of brassinosteroid has been revealed, how brassinosteroid functions in plants has not been elucidated. Therefore, elucidation of the function has been awaited from the viewpoints of basic research and applied research. The BIL1 gene was identified as the causative gene for the bil1 (brz-insensitive-long hypocotyl1) mutant having a longer hypocotyl than that of the wild-type plant (Non-Patent Documents 1 and 2) as a result of chemical biology research using “brassinazole (Brz)” which is an inhibitor capable of freely regulating biosynthesis of brassinosteroid. Thereafter, the BIL1-derived protein, i.e., the “BIL1 protein,” was found to be a transcription factor that regulates expression of many types of genes (approximately 3,000 types) among approximately 30,000 types of genes on the genome of Arabidopsis used as an experimental plant. At present, it is considered that the BIL1 protein is the master transcription factor (i.e., a factor that causes a chain reaction of gene expression) of brassinosteroid signaling.
The BIL1 protein is important in view of the wide range of subjects of transcription regulation. In addition, it is known to show interesting in-depth dynamics, by which the BIL1 protein is transferred into the nucleus from the cytosol with the addition of brassinosteroid. However, the detailed mechanism of regulation of transfer into the nucleus has not been elucidated.